1. Field of the Invention
The present invention relates to an active laser imaging system using a linear detector array (also called "line-scan array"). This system allows to observe a wide field and can, for example be used as an aid to the control of an aircraft by permitting a display of the ground located in front of the vehicle.
2. Description of the Prior Art
In a laser imaging system, the illuminator transmits a beam with a low divergence, of the order of 10.sup.-3 radian in general, that illuminates a portion of the scene. The back-scattered flux is collected by a receiving unit generally equipped with a single detector and whose field of view is matched to and of the same order of magnitude as that of the illuminator.
The system is equipped with an opto-mechanical scanning device that moves angularly and by one and the same value the direction pointed to by the illuminator and that pointed to by the receiving unit so as to cover point by point the desired scanning field. The detected signal is processed and then applied, for example, to a display device that reconstructs point by point the image of the scanned zone of space. Here the image can be different from the conventional television pictures and represent characteristics of the scene such as Doppler velocity of each point, or relief, or the display of obstacles, etc.
The field characteristics of these laser imaging systems can differ but the invention relates in particular to an application with a wide field, typically a few tens of degrees. The overlap can be only partial so as to define, for example, a meshing in the field, or alternatively to scan only a small number of lines (line-scan technique).
The utilization of these imaging systems reveals a detection problem related to the scanning angular velocity w and the distance D of the illuminated target. The laser flux transmitted by the illuminator propagates at the velocity of light c toward a point of the target; the same is true for the laser radiation backscattered toward the detector of the receiving unit. The direction pointed to by the receiving unit, at the time it receives the flux, no longer coincides with the direction pointed to by the illuminating system at the time of start of the pulse.
This angular shift .DELTA..alpha. is variable with the distance of the point which is pointed to and the instantaneous angular velocity of scanning. Note precisely, this shift is given by the relation: EQU .DELTA..alpha.=w.2(D/c)
In the French patent application FR-A-2 568 688, there is described a method for remedying these disadvantages by providing the imager with adjustable optical deflection means allowing to produce a predetermined relative angular shift between the pointing directions of the illuminator and the receiver so as to compensate for the scanning rotation .DELTA..alpha. that occurs during the to and fro travel of the laser luminous flux.
This technique is well adapted to the cases where the distances of the various points of the field to the imager are little different from each other. As a matter of fact, the correction is perfect only for a given mean adjustment distance and remains sufficient in a limited distance interval that cannot exceed a few hundred meters.
In the case of a control system with a wide field, the distances of the various points illuminated during a scan are very variable, for example from a few hundred meters to a few kilometers. Moreover, the requirement for a wide field usually leads to increase the angular velocity of scanning, which makes the correction still more sensitive to distance. As a matter of fact, if .theta. is the instantaneous angular field (zone of space seen by a photodetector of the receiver), the correction is sufficient for a distance interval .DELTA. D only if we have the relation: EQU w.(2/c)..DELTA.D&lt;.theta. or .DELTA.D&lt;.theta..c/2w
which means that the transmission-reception angular shift must remain smaller than the instantaneous field .theta. of the receiver. As a concrete example, we shall assume that the angular scanning speed w is such that the beam transmitted by the illuminator moves angularly by an angle .theta. in 5 microseconds. The distance interval for which the correction described in the above-mentioned patent application remains sufficient is therefore only of: EQU .DELTA.D=(c/2).5.times.10.sup.-6, that is 750 m.
This sufficient interval in the case of a small-field imager (small differences in depth of the various points of the field) is no longer sufficient in the case of a control system, when obstacles may appear suddenly at very variable distances and in a wider field.